Vivaldi antenna

ABSTRACT

There is provided a directional, wideband, planar antenna arrangement. The antenna arrangement is a class of Vivaldi aerial constructed as a plurality of conductive layers disposed on at least one substrate layer. The conductive layers are arranged to form a flared notch. The flared notch widens from a closed end to an open end. Instead of conforming to a simple exponential flare-shape, the inventive flared notch is arranged to conform to a hybrid curve. The hybrid curve comprises a plurality of self-similar curve sections. As the flare widens, each successive curve section is scaled up by a scaling factor and joined at its wider end with a neighbouring curve section. The antenna arrangement thereby becomes operable over a far wider frequency range than the conventional Vivaldi aerial. The hybrid flared notch can also be implemented in antipodal and balanced antipodal Vivaldi aerials.

The present invention relates to improvements in antennas. In particularthe present invention relates to broadband antenna of the Vivaldi, notchor tapered slot antenna family.

The Vivaldi antenna element was proposed by Gibson in 1979, (P. J.Gibson, The Vivaldi Aerial, in Proc. 9_(th) th European MicrowaveConference, UK, June 1979, pp.101-105). The original Vivaldi antennaswere tapered notch antennas having notches which open in an exponentialflare shape. They were constructed by conventional microwavelithographic thin film techniques on substrates having a high dielectricconstant, for example, alumina. Gibson's work has subsequently developedto include high gain Vivaldi antennas constructed on ceramic substratesother than alumina which have high dielectric constants and onsubstrates having low dielectric constant, for example, plastics.Copper-clad plastics (cuclad), for example PTFE, RT/duroid (having avariety of values, typically ε_(r)=2.2 or 2.94) or Kapton (ε_(r)=3.5),are now conventionally used when ease of manufacture, surface adhesionand price are paramount. Alternatively conductive layers can be formedfrom other good conductors including gold and gold-plated copper.

The exponential flare shape was originally adopted to address arequirement for a constant beamwidth antenna which could cover themicrowave frequency range between 2 GHz and 20 GHz. As Gibson explainsin his paper, the shape taken by the edge of the tapered slot must becompletely specified in terms of dimensionless normalised wavelengthunits for the beamwidth to be held constant. Exponential curves are goodcandidates for shapes specified in this way.

Approximations to constant beamwidth antennas can also be constructedusing alternative types of curves in place of exponential curves; thesealternatives include sinusoidal, parabolic, hyperbolic and polynomialcurves. The edges of the slot can also be formed as straight lines inwhich case the antenna can also be called a longitudinal (or linear)tapered slot antenna (LTSA).

Any conventional tapered slot antenna is constructed from a thinconductive layer disposed by lithographic thin film techniques on asubstrate. A slot, open at one end, (also known as a notch) is formed inthe conductive layer and the gap between the sides of the slot widensfrom a minimum at the closed end of the slot, also known as a “stub”, toa maximum at the open end. In conventional Vivaldi antennas, the gap ismirror-symmetrical about an axis through the centre of the slot and eachside of the conductive layer flares according to a predeterminedexponential formula. The flared slot is an effective radiating element.

In operation, the antenna radiates preferentially from the open end ofthe notch in a direction away from the notch and along the axis ofsymmetry. The antenna may thus be classed as an endfire antenna.

Each region of conductive layer having a flare shaped edge willhenceforth be referred to as a wing of the antenna due to the appearanceof the conductive layer. It has been found effective to dispose twopairs of mirror-symmetrical wings on a thin substrate layer: one pair oneither planar surface of the substrate layer. The pairs are preferablyidentical and the notch formed by one pair is preferably disposedparallel to the notch formed by the other pair.

The closed end of the slot line may be fed by any one of a variety oftransmission lines including microstrip lines, striplines, fin-lines (asin waveguides) and probes. A microstrip transmission line generallycomprises a track of conductor (usually copper) on an insulatingsubstrate. On the reverse side of the substrate there is formed a groundplane (or “backplane”) of conductor which acts as the return conductor.

Certain arrangements of tapered slot antenna can be fed from twoparallel strips of conductor on either surface of a flattened substratein a transmission line formation know as a twinline feed. Variations onthe Vivaldi antenna structure for which a twinline feed is appropriateinclude the (unbalanced) antipodal Vivaldi antenna and the balancedantipodal Vivaldi antenna.

In twinline fed antennas, the conductive wing regions are each arrangedto have an inner edge and an outer edge. In the same way as the edge ofthe slot in a conventional Vivaldi antenna follows a flared curve, theinner edge of the conductive wing regions can be formed to conform to asimilar flared curve. In contrast to the indefinite extent of theconductive layer away from the slot in a conventional Vivaldi antennaarrangement, a second outer edge can define the outer extent of eachconductive wing. The outer edge too can be formed to follow a broaderflared curve.

The (unbalanced) antipodal Vivaldi antenna was developed by Gazit in1988 (E. Gazit, Improved design of the Vivaldi antenna, in IEE Proc.,Vol. 135, Pt. H, No. 2, April 1988, pp 89-92) is constructed on a singlesheet of microwave dielectric substrate and fed from a twinline. Theconductor strip on one side of the twinline feeds a first wing on afirst side of the substrate and the other conductor strip feeds a secondwing on the second side of the substrate. The first and second wings arearranged so that, from a point of view at right angles to the plane ofthe substrate, there is a flare shaped slot.

The balanced antipodal Vivaldi antenna, developed by J. D. S. Langley,P. S. Hall and P. Newham in 1996, is constructed on a sandwich of atleast two sheets of dielectric substrate and fed from a balancedtwinline.

A balanced antipodal Vivaldi antenna can be constructed from a firstwing on one side of a first sheet of dielectric substrate and a secondwing on the other side of the first sheet. A second sheet of dielectricsubstrate is provided with a third wing on an outer side. The firstsheet and second sheet are sandwiched together so that the first andthird wings are outermost and so that a sheet of dielectric substrate isinterposed between the first wing and the second wing and between thethird wing and the second wing. The first and third wings are arrangedto flare in a first curved shape. The second wing is arranged to flarein a second curved shape—the second curved shape being the mirror imageof the first curved shape. When viewed at right angles to the plane ofthe substrates, the first and third wings on one side and the secondwing on the other side form a flare shaped slot.

In theory, a Vivaldi antenna should radiate radio frequencyelectromagnetic waves at a given wavelength when the width of thewidening slot (at right angles to the axis of symmetry) is approximatelyequal to half the wavelength. The performance of physicalimplementations of conventional antennas is degraded by a number ofcomplicating factors. In particular, the edge of the flared slot becomeslinear at either extreme of a limited range of frequencies.

It has been established experimentally that the conventional exponentialflare shaped Vivaldi antenna has poor performance over ultra-widebandwidths. The crisp radiation properties of the exponential flarebreak down both as operating frequency increases above the bounds of acharacteristic range and as the frequency decreases below the bounds.

It has been noted that antennas constructed to the same basicexponential curve have a most reliable frequency range which dependsupon the characteristic length scale of the antenna. To give concreteexamples, an antenna having a maximum flare width of two centimetres hasa relatively reliable performance over the frequency range 15-40 GHzwhile a larger antenna with a maximum flare width of the order of tencentimetres has a better performance at lower frequencies, between 1 and1 OGHz. In the example, the dielectric constant of the substrate used inboth antennas was 2.94.

A perfect antenna would radiate electromagnetic waves of a givenfrequency at a point along the centre line of the slot for which thewidth of the widening slot is equal to half the wavelength correspondingto the given frequency. In the real world, antennas do not function sostraightforwardly. As the given frequency increases, the point ofradiation moves towards the closed end of the slot. As the slot narrows,the gradient of the exponential curve of the slot edge decreases in thedirection of the closed end and becomes too shallow to radiateeffectively.

On the other hand, as the given frequency decreases, the point ofradiation moves towards the open end of the slot. As the slot becomeswider the gradient of the exponential curve increases and becomes toosteep to radiate effectively.

It is therefore an object of the invention to obviate or at leastmitigate the aforementioned problems.

In accordance with one aspect of the present invention, there isprovided a planar antenna arrangement for emitting electromagnetic wavesin an endfire direction, the antenna arrangement comprising: a pluralityof conductive layers; and at least one substrate layer, wherein theconductive layers are arranged to form a notch, the notch having aclosed end and an open end and the endfire direction being the directionfrom the closed end to the open end, wherein each conductive layercomprises at least one conductive wing, each conductive wing boundingthe notch at an inner edge, and wherein the inner edge of eachconductive wing is arranged to conform to a hybrid curve, the hybridcurve comprising a plurality of curve sections.

Advantageously, the hybrid curve is monotonically increasing in theendfire direction.

Each of the curve sections may be a section of an exponential curve.

Preferably, the curve sections are self-similar. Every self-similarcurve section may conform to a corresponding curve formula, the curveformula corresponding to adjacent curve sections differing by afundamental scaling factor; and the self-similar curve sections mayincrease in scale as the notch widens towards the open end, whereby eachcurve section disposed closer to the open end of the notch is scaled upby the fundamental scaling factor from each adjacent curve sectiondisposed closer to the closed end of the notch.

It is preferred that the hybrid curve comprises a first curve sectionand a second curve section, one end of the first curve section beingdisposed at the closed end of the notch, the remaining end of the firstcurve section meeting with one end of the second curve section at afirst node and the second curve section having the same curved form asthe first curve section.

The hybrid curve may comprise a further curve section, said furthercurve section meeting the remaining end of the second curve section at afurther node and having the same curved form as the first and secondcurve sections.

The hybrid curve may comprise yet further curve sections, the or each ofsaid further curve sections meeting a remaining end of each respectivepreceding curve section at yet further nodes and having the same curvedform as the first and second curve sections.

Advantageously, the or each of said nodes may be blended to eliminatediscontinuities.

Each successive curve section is preferably longer in the endfiredirection than each respective preceding curve section.

The conductive layers may advantageously be fed by a microstriptransmission line.

Alternatively the conductive layers may be fed by a twinline. Theantenna may be an antipodal antenna. The antenna may also be a balancedantipodal antenna. In either case, the trailing edge of each conductivewing is advantageously arranged to conform to a further hybrid curve.

The present invention addresses problems associated with the exponentialflare shape used in known Vivaldi antennas by adopting a curved shapewhich conforms to a hybrid curve. When the hybrid curve is constructedfrom a succession of self-similar curve sections flare shape can be saidto be fractalized.

For a better understanding of the present invention, reference will nowbe made, by way of example only, to the accompanying drawings in which:

FIG. 1 is a diagram of an exponential curve suitable for a conventionalVivaldi antenna;

FIG. 2 shows a conventional microstrip transmission line;

FIG. 3A shows an arrangement of conductive wings suitable for use in aconventional Vivaldi antenna;

FIG. 3B shows a conventional Vivaldi antenna arrangement;

FIG. 4 shows a conventional unbalanced antipodal Vivaldi antennaarrangement;

FIG. 5 shows a conventional balanced antipodal Vivaldi antennaarrangement;

FIG. 6A shows an arrangement of conductive wings suitable for use in aVivaldi antenna arrangement in accordance with the present invention;

FIG. 6B shows an alternative arrangement of conductive wings suitablefor use in a Vivaldi antenna arrangement in accordance with the presentinvention;

FIGS. 7A to 7E show examples of blended and unblended exponential curveswhich may define the edge curve of conductive wings in accordance withthe present invention;

FIG. 8 shows a Vivaldi antenna arrangement in accordance with IS theinvention;

FIG. 9 shows an unbalanced antipodal Vivaldi antenna arrangement inaccordance with the invention; and

FIG. 10 shows a balanced antipodal Vivaldi antenna arrangement inaccordance with the invention.

FIG. 1 is a diagram of an exponential curve 120 and can be used toillustrate how a conventional Vivaldi antenna operates over a range offrequencies. A conventional Vivaldi antenna includes a conducting layercomprising two symmetrical conducting wings. Each of the conductingwings has an inner edge which is cut away along an exponential curve. Aflared notch is thereby formed between the two conducting wings. Radiofrequency waves at a given frequency radiate from a corresponding pointalong the axis of symmetry, X. The corresponding point is the point atwhich the width of the flared notch is equal to half the wavelength.

In principle, increasingly higher frequencies are radiated from pointsincreasingly closer to the left of the illustrated exponential curve.Effective radiation is limited at both a lower and an upper frequencyboundary, 112,114.

As the given frequency increases, the corresponding point of radiationmoves towards the closed end of the flared notch. From points to theleft of the lower boundary 112, the flared notch narrows so much thatthe gradient of the exponential curve 120 becomes too shallow to radiateeffectively.

As the given frequency decreases, the corresponding point of radiationmoves towards the open end of the flared notch. For points to the rightof a second boundary 114, the notch becomes so wide that the gradient ofthe exponential curve becomes too steep to allow effective radiation.

An appropriate feeding mechanism for certain antenna in accordance withthe present invention would be a microstrip transmission line. As may beseen in FIG. 2, a microstrip transmission line generally comprises atrack of conductor 220 (usually copper) on an insulating substrate 240.On the reverse side of the substrate 240 there is formed a ground plane230 (or “backplane”) of conductor which acts as the return conductor.

FIGS. 3A to 5 show arrangements of different conductive wings suitablefor use in a conventional antennas. FIG. 3B shows a conventional Vivaldiantenna arrangement. FIGS. 4 and 5 show conventional unbalanced andbalanced antipodal Vivaldi antenna arrangements respectively.

FIG. 3A shows the pattern in which one conductive layer is disposed upona substrate in the construction of conventional Vivaldi aerial 300. Anotch 316 is formed in the conductive layer and the gap between thesides of the slot (the two ‘wings’) widens from a minimum 312 at theclosed end of the notch to a maximum 318 at the open end. The gap ismirror-symmetrical about an axis 314 through the centre of the notch 316and each side 304,306 of the conductive layer flares according to apredetermined exponential formula.

As may be seen from FIG. 3B, a Vivaldi aerial may be constructed fromtwo pairs of mirror-symmetrical wings 304,306,304′,306′ on a thinsubstrate layer 310: one pair on either planar surface 320,330 of thesubstrate layer 310. The pairs 304,306,304′,306″ are preferablyidentical and the notch 316 formed by one pair is preferably disposedparallel to the notch 316′ formed by the other pair.

The antennas 300 in FIG. 3 are fed by a transmission line, such as themicrostrip line illustrated in FIG. 2, at the closed end of the notch302.

As discussed above, the class of Vivaldi antennas includes antipodalVivaldi antenna, both unbalanced and balanced. Examples of antipodalVivaldi antennas are shown in FIGS. 4 and 5.

In antipodal Vivaldi antennas, the conductive wing regions404,406,504,506,508 are each arranged to have an inner edge 414 and anouter edge 412. Just as the edge of each wing 304,306 in FIG. 3A followsa flared curve, the inner edge 414 of the conductive wing regions ofFIGS. 4 and 5 can be formed to follow a similar flared curve. Incontrast to the indefinite extent of the conductive layer away from theslot in the conventional Vivaldi antenna arrangement 300, an outer edge412 can define the outer extent of each conductive wing. The outer edge412 too can be formed to follow a broader flared curve.

As shown in FIG. 4, the unbalanced antipodal Vivaldi antenna 400 isconstructed on a single sheet of microwave dielectric substrate 410 andfed from a twinline 402. The conductor strip on one side of the twinlinefeeds a first wing 406 on a first side 430 of the substrate and theother conductor strip feeds a second wing 404 on the second side 420 ofthe substrate. The first and second wings 404,406 are arranged so that,from a point of view at right angles to the plane of the substrate 410,there is a flare shaped slot 416.

In a similar manner the balanced antipodal Vivaldi antenna 500 shown inFIG. 5 is constructed on a sandwich of at least two sheets of dielectricsubstrate 510, 550 and fed from a balanced twinline 502.

A balanced antipodal Vivaldi antenna 500 can be constructed from a firstwing 506 on one side 530 of a first sheet of dielectric substrate 510and a second wing 504 on the other side 520 of the first sheet 510. Asecond sheet of dielectric substrate 550 is provided with a third wing508 on an outer side 560. The first sheet 510 and second sheet 550 aresandwiched together so that the first and third wings 506,508 areoutermost and so that a sheet of dielectric substrate is interposedbetween the first wing 506 and the second wing 504 and between the thirdwing 508 and the second wing 504. The first and third wings 506,508 arearranged to flare in a first curved shape. The second wing 504 isarranged to flare in a second curved shape—the second curved shape beingthe mirror image of the first curved shape. When viewed at right anglesto the plane of the substrates, the first and third wings on one sideand the second wing on the other side form a flare shaped slot 516.

The range over which conventional Vivaldi antenna can operate is limitedby the phenomena discussed in relation to FIG. 1. It has been found thatby constructing the flare shaped notch to conform to a certain hybridcurve the range over which an antenna can operate can be vastlyincreased.

FIGS. 6 and 7 illustrate how such a hybrid curve should be constructed.As may be seen in FIGS. 6A and 6B, the curve is composed of two or moresmaller curves. The smaller curves can belong to a variety of categoriesincluding exponential, sinusoidal, and parabolic. FIGS. 6A and 6B showversions of an antenna. In both cases the antenna is fed from a slotline. The curve in FIG. 6A is formed from a hybrid of two exponentialcurve sections 602,602′. Similarly, the curve in FIG. 6B is formed froma hybrid of four exponential curve sections 604,604′,604″, 604′″.

It will be noted from FIG. 6B that each successive curve section604,604′,604″,604′″ is similar to its neighbour but scaled by a scalingfactor. In cases where curve sections are scaled versions of theirneighbours it is appropriate to call the hybrid curve a fractal, orfractalized, curve and the individual curve sections may be termedself-similar.

The embodiments of such fractalized flare shapes described herein areexample only, the numbers of curve sections in each hybrid curve, theform taken by each curve section, and the scaling factor will clearly bevaried in accordance with the requirements of any particularimplementation.

The same hybrid curves 610, 620 are shown at FIGS. 7B and 7Drespectively. To overcome problems that may be associated with the sharpdiscontinuities (such as a null in the boresight gain pattern atspecific frequencies) the curves that comprise hybrid curves may beblended to some degree. Examples of blended curves are shown at FIGS.7A, 7C and 7E.

In FIG. 7C the hybrid curve 610 formed from two exponential curvesections is shown partially blended 706. This contrasts with a fullyblended version 702 shown at FIG. 7A. The sharp discontinuity 710 isblended away to leave an inflection point 712.

FIG. 7E shows a partially blended version 710 of the hybrid curve 620 inFIG. 7D. Again sharp discontinuities are avoided.

As will be appreciated the proposed improvements to the curved shapes ofthe inner sides of conductive wing regions apply equally to conventionalVivaldi antenna, unbalanced antipodal Vivaldi antenna and balancedantipodal Vivaldi antenna.

FIG. 8 shows a Vivaldi antenna arrangement 800 in accordance with thepresent invention. The antenna is fed by a slot line 802 and isconstructed from a single sheet of double sided copper clad dielectricsubstrate 810.

In this first embodiment of the present invention, the hybridfractalized curve 620 constructed from four exponential curve sectionsis implemented on the inner edge of the wing regions 804,806, 804′,806′.

The antenna arrangement shown in FIG. 9 is also constructed from asingle sheet 910 of double sided copper clad dielectric substrate. Onthis occasion the antenna is fed by a twinline 902.

FIG. 9 shows a second embodiment of the present invention in which thehybrid fractalized curve 620 is applied to the inner edges 914 of theconductive wing regions 904,906 in an unbalanced antipodal configuration900.

It is noted that the trailing edges 912 of the conductive wing regionsare also formed in accordance with a hybrid fractalized curve.Furthermore the series of curve sections making up the fractalizedtrailing edge 912 may be blended as described in FIGS. 7A to 7E. The useof hybrid curves on the trailing edge 912 can help reduce low frequencyreturn loss.

The balanced antipodal Vivaldi antenna shown in FIG. 10 is constructedfrom two sheets of double sided copper clad dielectric substrate1030,1050 sandwiched together and is fed from a balanced twinline 1002.

FIG. 10 shows a third embodiment of the present invention in which thehybrid fractalized curve 620 is applied to the inner edges 1014 of theconductive wing regions 1004,1006 in a balanced antipodal configuration1000.

Again the trailing edges 1012 of the conductive wing regions 1004,1006are also formed in accordance with a hybrid fractalized curve.

As will be understood antennas in accordance with the present inventionmay constructed from a conductor clad dielectric microwave substratematerial just as conventional Vivaldi antennas are. The type ofconstruction depends upon the type of feed to the antenna which in turndepends upon the particular class of antenna implemented.

The foregoing discussion considered the arrangement of a single antenna.It is however well known in the art to form arrays from a plurality ofsimilar antennas. Furthermore it is known to provide antennas withidentical endfire directions but rotated at an angle relative to oneanother about the endfire axis to allow for radiation having differentpolarisation. It will be understood that antennas in accordance with thepresent invention can be used as elements of an antenna array and inorthogonal pairs for dual-polarised functionality. The present inventionis also considered applicable to arrays of dual-polarised antenna pairs.

1. A planar antenna arrangement for emitting electromagnetic waves in anendfire direction, the antenna arrangement comprising: a plurality ofconductive layers; and at least one substrate layer, wherein theconductive layers are arranged to form a notch, the notch having aclosed end and an open end and the endfire direction being the directionfrom the closed end to the open end, wherein each conductive layercomprises at least one conductive wing, each conductive wing boundingthe notch at an inner edge, and wherein the inner edge of eachconductive wing is arranged to conform to a hybrid curve, the hybridcurve comprising a plurality of curve sections.
 2. An antennaarrangement according to claim 1, wherein the hybrid curve ismonotonically increasing in the endfire direction.
 3. An antennaarrangement according to claim 1, wherein each of the curve sections isa section of an exponential curve.
 4. An antenna arrangement accordingto claim 1, wherein the curve sections are self-similar.
 5. An antennaarrangement according to claim 4, wherein every self-similar curvesection conforms to a corresponding curve formula, the curve formulacorresponding to adjacent curve sections differing by a fundamentalscaling factor; and wherein the self-similar curve sections increase inscale as the notch widens towards the open end, whereby each curvesection disposed closer to the open end of the notch is scaled up by thefundamental scaling factor from each adjacent curve section disposedcloser to the closed end of the notch.
 6. An antenna arrangementaccording to claim 1, wherein the hybrid curve comprises a first curvesection and a second curve section, one end of the first curve sectionbeing disposed at the closed end of the notch, the remaining end of thefirst curve section meeting with one end of the second curve section ata first node and the second curve section having the same curved form asthe first curve section.
 7. An antenna arrangement according to claim 6,wherein the hybrid curve comprises a further curve section, said furthercurve section meeting the remaining end of the second curve section at afurther node and having the same curved form as the first and secondcurve sections.
 8. An antenna arrangement according to claim 6, whereinthe hybrid curve comprises yet further curve sections, the or each ofsaid further curve sections meeting a remaining end of each respectivepreceding curve section at yet further nodes and having the same curvedform as the first and second curve sections.
 9. An antenna arrangementaccording to claim 6, wherein the or each of said nodes is blended toeliminate discontinuities.
 10. An antenna arrangement according to claim6, wherein each successive curve section is longer in the endfiredirection than each respective preceding curve section.
 11. An antennaarrangement according to claim 1 wherein the conductive layers are fedby a microstrip transmission line.
 12. An antenna arrangement accordingto claim 1, wherein the conductive layers are fed by a twinline.
 13. Anantenna arrangement according to claim 12, wherein the antenna is anantipodal antenna.
 14. An antenna arrangement according to claim 13,wherein the antenna is a balanced antipodal antenna.
 15. An antennaarrangement according to claim 13, wherein the trailing edge of eachconductive wing is arranged to conform to a further hybrid curve. 16.(Cancelled)